Modem control method

ABSTRACT

When carrying out pre-processing of image signal transmission using the control channel of a full-duplex modem and switching image signal transmission to the primary channel of a half-duplex modem, the answer modem stops the transmission carrier upon confirmation that all “1” signals with 40 bit or more have been received from the calling modem, restarts the reception of all “1” signals after the elapse of a certain period of time, confirms the end of all “1” signals and switches to the primary channel.

TECHNICAL FIELD

The present invention relates to modem control methods when afull-duplex modem is switched to a half-duplex modem. Particularly, itrelates to control methods of a V.34 facsimile modem which uses a V.34Recommendation modem established at the SG14 conference of the ITU(International Telecommunication Union) held in June 1994 andcommunicates through a T.30 Recommendation ANNEX-F communicationprocedure approved by mail voting at the SG8 conference of the ITU heldin July 1996.

BACKGROUND ART

The communication protocol based on the current V.34 Recommendation andT.30 Recommendation ANNEX-Fuses a full-duplex modem up to the controlchannel and a half-duplex modem for the primary channel which transmitsimage information. In this way, it switches over between these modems.FIG. 1 shows the communication protocol.

In FIG. 1, phase 1 is a section of exchange between a CM signal (callingmenu signal) and JM signal (common menu signal). This section selects amodulation mode available to the calling and answer modems. Phase 2 is asection called “line probing.” L1 and L2 each includes simultaneoustransmissions of 21 single-frequencies from 150 Hz to 3750 Hz, and areused to probe the amplitude characteristic of the line viewed from theanswer modem. INFO is a communication capacity information signal and A,A, B, and B indicate the ACK (Acknowledge) signals that receive INFO.

Phase 3 is a primary channel preparation stage which transmits V.34image signal data which will be described later and corresponds to theperiod of transmission of a long training signal (long synchronizationsignal). The frequency band (or symbol rate) is determined based on theprobing result of the line amplitude characteristic of the L1 and L2signals in phase 2.

The control channel includes signals represented by C and D. Part C isthe section in which parameters for the modem itself are mainlyexchanged, determining the signal rate of the image signal datatransmitted based on a modulation system called “primary channel” whichcomes later. Part D is a section to exchange control information as thefacsimile terminal and is controlled according to commands such as DIS(Digital Identification Signal) and DCS (Digital Command Signal)described in the T.30 Recommendation.

A signal called “MPh (Modem Parameter Exchange)” of part C contains abit (bit 50 in MPh) which determines whether or not to accept asymmetrictransmission rates of 1200 bps and 2400 bps as the transmission rate ofpart B in the same control channel and is further provided with anotherbit (bit 27 in MPh) which requests the answer modem to transmit part Bat either 1200 bps or 2400 bps.

At present, the T.30 ANNEX-F does not accept asymmetric transmissionrates in FAX transmissions by setting MPh bit 50 to “0”. When asymmetric rate transmission is selected, if the requested transmissionrate differs between the calling and answer modems, transmission of partD is designed to be carried out according to the lower rate side. In acurrent FAX which incorporates a V.34 modem, the transmission rate ofpart D of this control channel is preset to either 1200 bps or 2400 bps.

The primary channel includes facsimile image information data which istransmitted by a modulation system called “V.34 primary channel” and ashort training signal (short synchronization signal) which precedes thefacsimile image information data. The data signal rate of this imageinformation part is determined by the transmission/reception of theaforementioned MPh signal of part C of the control channel.

FIG. 2 is a flowchart that shows a switchover from the end of thecontrol channel program to the primary channel on the answer modem. FIG.2 shows the operation of the control channel in FIG. 1 when the answermodem transmits a CFR signal (CONFIRMATION TO RECEIVE signal: based onT.30 Recommendation), then a Flag pattern (a series of “7E”s inhexadecimals: based on T.30 Recommendation) and waits for all “1”signals of at least 40 bits from the calling modem.

Since each symbol of the control channel of the V.34 Recommendationconsists of 2 bits or 4 bits, the answer modem normally judges data forevery short bits such as 2 bits, 4 bits or 8 bits. Therefore, it judgeswhether the data is all “1” or not for every 8 short bits (S1).Furthermore, it judges whether all “1” receive signals consist of atleast 40 bits or not according to the ANNEX-F of the T.30 Recommendation(S2). When it is confirmed that the receive signal contains a series of“1”s for at least 40 bits, the Flag signal that was being transmitted iscut and transmitted with a turn-off sequence of the V.34 Recommendation(all “1” signal in a short period) added and then signal transmission isstopped (S3).

In response to this, the calling modem waits until the carrier from theanswer modem turns off, and stops transmission of all “1” signals, addsa turn-off sequence, then turns off signal transmission. The answermodem monitors the carrier from the calling modem turn off (S4) and ifit confirms that this carrier turns off, then closes the reception ofthe control channel and switches to the reception of the primary channel(S5).

However, since the switching method above switches the reception modeafter detecting that the carrier from the calling modem turns off, ithas the problem of easily missing the switching timing. In addition,this method of switching to the receive mode after detecting that thecarrier turns off, no matter how secure it may appear, actually has adisadvantage of the modem having difficulty in catching the signal-offsection if line noise of −43 dBm or greater exists on the line. Withoutdetecting this signal-off section, the switching of the primary channelis impossible, making it impossible to receive image information data.The T.4 Recommendation stipulates that a facsimile apparatus shouldoperate normally when the receive signal level ranges 0 dBm to −43 dBmand the transmit output should be made adjustable from −15 dBm to 0 dEm.

One of the methods to switch from the control channel to the primarychannel is a method in which the answer modem continues reception anddemodulation irrespective of whether the carrier from the calling modemturns on/off, detects the timing at which all “1” receive data ischanged to data other than all “1” data and then switches to thereception of the primary channel. However, if a considerable amount ofdelay is introduced into the line, this system has the problem ofprovoking switching timing errors as shown below in the case ofcommunications with overseas for instance, resulting in a failure in thereception of the primary channel.

FIG. 3 is a timing chart that shows the terminating timing of thecontrol channel assuming that a delay of 50 ms has occurred. If there isno line delay, the answer modem turns off its carrier when it confirms40 bits of all “1” signals from the calling modem, and thus the callingmodem also detects the carrier-off and stops transmission of all “1”.The answer modem detects the stoppage of transmission and switches tothe primary channel. This switching is performed with the timing of E.

In the case of a delay of 50 ms, if the answer modem confirms 40 bits ofall “1” signals and turns off the carrier, the calling modem detects it50 ms after point E and turns off the transmission of all “1”. It is atpoint F, 50 ms later, that this transmission-off is detected on theanswer modem. Therefore, the answer modem switches to the primarychannel at this point F.

As shown above, if there is a large amount of delay in the circuit andgreat attenuation in both directions on the line, for example, if thereis a great level difference between the transmission level of −15 dBmand incoming level of −43 dBm, the answer modem is likely to erroneouslyswitch from the control channel to the primary channel with the timingof E.

FIG. 4 shows a transmission wraparound at the answer modem and thereceive signal from the calling modem. The signal having a width of “a”indicates an all “1” transmit signal from the calling modem and showsconsiderable attenuation due to circuit attenuation. The signal having awidth of “b” indicates a wraparound signal of the Flag signal on theanswer modem and is a large signal because it is not attenuated. Theratio b/a is as great as 20. If there is no circuit attenuation, “a”approximates to “b”. For example, if the transmit signal from the answermodem becomes a near-end echo of the exchange and returns to the answermodem without attenuation, the amplitude of the wraparound signal fromthe answer modem may grow approximately 20 times in size compared to theamplitude of the incoming signal from the calling modem at the input ofthe A/D converter.

With the timing of G in FIG. 4, the carrier is turned off after a 40-bitsignal is received, and therefore the transmission wraparound from theanswer modem is cut off. This reduces the amplitude considerably from“b” to “a”, making it easier to produce bit errors. That is, waiting forthe all “1” signal to terminate while simply checking the all “1” signalfrom a point just before G in FIG. 4 is likely to switch to thereception of the primary channel with the timing of G erroneously.Switching with this timing will receive the control channel signal (all“1” signal) in receive mode of the primary channel. This timing errorwill result in a failure in the reception of the primary channel.

DISCLOSURE OF INVENTION

The present invention has been implemented in view of the problemsabove. Its objective is to provide modem control methods which willassure the switching from a full-duplex modem to a half-duplex modemeven in the case that the line has a large delay and attenuation.

This objective is achieved by modem control methods in which the controlchannel of a full-duplex mode is switched to the primary channel of ahalf-duplex modem as follows: When an all “1” termination request signalof 40 bits or more which indicates a control channel termination requestfrom the calling modem is received, it is first confirmed that thesignal received from the calling modem is an all “1” 40-bit signal andthen transmission to the calling modem is stopped. The confirmation ofthe reception of the all “1” signal from the calling modem is ignoredfor a certain period of time, then the confirmation of the reception ofthe all “1” signal is restarted. When the termination of the receptionof the all “1” signal is confirmed, the control channel of thefull-duplex modem is switched to the reception of the primary channel ofthe half-duplex modem. This assures the switching from the controlchannel to the primary channel.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram showing the overall T.30 ANNEX-F procedurein a V.34 facsimile modem;

FIG. 2 is a flowchart showing the conventional method of switching thereception from the control channel to the primary channel;

FIG. 3 is a timing chart to explain the conventional method of switchingthe reception from the control channel to the primary channel;

FIG. 4 is a schematic diagram to explain conventional problems inswitching the reception from the control channel to the primary channel;

FIG. 5 is a schematic section diagram showing the main components of aV.34 control channel modem configuration of the present invention;

FIG. 6 is a section diagram to explain the control section of the modemshown in FIG. 5;

FIG. 7 is a diagram to explain the switching of reception from thecontrol channel to the primary channel in the control method of thepresent invention; and

FIG. 8 is a flowchart of the control method of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

In facsimile transmissions, the modem control methods of the presentinvention implement pre-processing of image signal transmission througha full-duplex modem and then switches to image signal transmissionthrough a half-duplex modem. When transmission through the full-duplexmodem is completed, if a termination request from the calling modem isto transmit a series of unit signals comprised of prescribed bits, aprescribed number of said unit signals are received from the callingmodem, then transmission to the calling modem is stopped, and thereception of said unit signals from the calling modem is ignored for acertain period of time, and then the reception of said unit signals isrestarted and upon completion of the reception of these unit signals theprocess is switched to the reception through the half-duplex modem.

If a termination request from the calling modem is to transmit a seriesof unit signals combining prescribed bits, the answer modem stopstransmission to the calling modem upon reception of a prescribed numberof these unit signals. In the meantime, the calling modem continues totransmit unit signals, but the answer modem ignores the reception ofthose unit signals for a certain period of time, after which it restartsreceiving unit signals. When it detects that transmission from theanswer modem has been completed, the calling modem stops transmittingunit signals thereafter. The answer modem detects termination of theseunit signals and switches from the full-duplex modem to the half-duplexmodem.

If there is no delay on the line, the time at which the answer modem hascompleted transmission, the time at which the calling modem hascompleted unit signals by this completion of transmission and the timeat which the answer modem detects this completion are almost the same,and therefore even if the answer modem switches from the full-duplexmodem to the half-duplex modem there will be no problem. However, ifsome delay is produced on the line, these three points will not coincideand especially the output of receive signals will change greatly whenthe answer modem completes transmission, which is likely to cause thispoint to be mistaken for the time of completion of unit signals.

The present invention stops the reception of signals during this periodsusceptible to mistaking for a prescribed period of time and restartsthe reception when receive signals are stabilized, detects the end ofunit signals correctly and switches to the half-duplex modem.

To be concrete, the present invention uses an ITU Recommendation V.34facsimile modem, carries out the pre-processing of image signaltransmission by the control channel of the full-duplex modem incompliance with the ITU Recommendation V.30 ANNEX-F. When switching tothe primary channel of the half-duplex modem for image signaltransmission, upon reception of a termination request signal of at least40 all “1” bits which indicates a request for termination of the controlchannel from the calling modem, it first confirms that the signalreceived form the calling modem is a signal of at least 40 all “1” bits,and stops the transmission to the calling modem, then ignores thereception of all “1” signals from the calling modem for a certain periodof time, after which it restarts the reception of all “1” signals, andafter confirming the end of the reception of the all “1” signal,switches to the primary channel.

In other words, the calling modem transmits a series of all “1” bits ofthe control channel termination request and when 40 bits or more arereceived the answer modem stops the transmission to the calling modem,and after the elapse of a certain period of time it restarts thereception of all “1” signals. When the transmission from the answermodem is stopped, the calling modem stops the transmission of all “1”signals. The answer modem detects the stoppage of these all “1” signalsand switches to the primary channel.

If there is no delay on the circuit, the time after the answer modemstops the transmission until the stoppage of all “1” signals from thecalling modem is detected is short, and thus there will be no problemeven if the answer modem switches to the primary channel a short timeafter the transmission is stopped. However, if there is some delay onthe line, the time after the answer modem stops the transmission untilthe stoppage of all “1” signals from the calling modem is detected willincrease. The signal received by the answer modem contains a largepercentage of echoes of the signal sent by itself and if the answermodem stops the transmission, this echo attenuates and the receive levelis reduced with the result that this reduction of the receive level islikely to be mistaken for the stoppage of all “1” signals from thecalling modem causing the answer modem to switch to the primary channel.

The control method of the present invention stops the reception of all“1” signals during this period susceptible to errors and restarts thereception after the receive signals are stabilized. Thus, it can detectthe end of all “1” signals correctly and switch to the primary channelcorrectly. Large attenuation on the line may further increase variationsof the signal level when an echo disappears, while it may reduce thereceive level of all “1” signals after the answer modem stops thetransmission. However, the control methods of the present inventionreceive all “1” signals in a period in which the receive signals arestabilized, providing a reliable method for the detection of the end ofall “1” signals.

Furthermore, the control methods of the present invention set theoperation of an AGC (Automatic Gain Control) circuit which adjusts thegain of the receive signals to fixed mode starting immediately beforethe confirmation of the reception of all “1” signals from the callingmodem is ignored for a given time until the switching to the receptionof the primary channel is completed.

The AGC circuit that adjusts the gain of receive signals is normally setto variable mode and if the receive level is reduced, it tries torestore this to the original level to maintain a certain level. However,if this is changed to fixed mode, the AGC stops its function and whenthe receive level reduces, it retains that low level. When the signallevel changes, trying to restore this to the original level is likely tocause bit errors. Thus, the answer modem sets the AGC to fixed modeduring a period in which the signal level is prone to change, anddetects the time of the end of all “1” signals correctly and switches tothe primary channel correctly.

With reference now to the attached drawings, the embodiment of thepresent invention is explained below.

FIG. 5 is a section diagram showing the main components of the answermodem of the modem of the present invention. This modem mainly comprisesmodulator 11 that modulates data, D/A converter 12 that digital/analogconverts the modulated data, hybrid circuit 13 that switchestransmission direction in two directions, transformer 14, switch 15 thatswitches transmission and reception, A/D converter 16 thatanalog/digital converts signals from the calling modem, demodulator 17that demodulates the converted data and control section 18 that controlschannel switching.

In the configuration above, a signal is modulated by modulator 11, D/Aconverted by D/A converter 12 and this signal is transmitted from switch15 to the calling modem. A signal from the calling modem is receivedthrough switch 15, A/D converted by A/D converter 16 and demodulated bydemodulator 17.

As shown in FIG. 6, control section 18 comprises signal confirmationsection 21 that confirms whether a signal from the calling modem is anall “1” signal or not, carrier-off indication section 22 that indicatestransmission carrier-off based on the output from signal confirmationsection 21, processing standby/restart indication section 23 thatcontrols indications of standby and restart of the processing based onthe output of carrier-off indication section 22 and switching indicationsection 24 that switches the reception from the control channel to theprimary channel based on the output from signal confirmation section 21.

When signal confirmation section 21 receives an all “1” signal andconfirms that there is a series of “1”s by a prescribed number of bits,control section 18 outputs a transmission carrier-off indication tocarrier-off indication section 22. Based on this signal, carrier-offindication section 22 indicates transmission carrier off. Based on thisindication, processing standby/restart indication section 23 ignores thereception of all “1” signals from the calling modem for a certain periodof time and holds the processing in standby mode. Then, after the elapseof a certain period of time, processing standby/restart indicationsection 23 indicates the restart of the reception of the all “1” signal.Based on this indication, if the receive signal is not an all “1”signal, signal confirmation section 21 indicates it to switchingindication section 24, which in turn indicates a switchover from thereception of the control channel to the reception of the primarychannel.

Then, the following illustrates the case where the control method of thepresent invention is carried out using a modem with the configurationdescribed above. FIG. 7 is schematic diagram to explain the controlmethod of the present invention. FIG. 8 is a flowchart showing thealgorithm of the operation in FIG. 7. Suppose a delay of 50 ms, forexample, has occurred on the line. Suppose the calling modem transmits aFlag signal made up of “7E” in hexadecimals, while the answer modemtransmits a CFR (Confirmation to Receive) signal. If the calling modemconfirms the CFR signal, it transmits an all “1” signal which is asignal with a series of “1”s.

Then, when the transmission of the CFR signal is completed, the answermodem transmits the 7E Flag signal while receiving the all “1” signal.It receives the all “1” signal, for example, in 8-bit units and confirmsthat they are “1”s (S11). In this case, signals are received in 8-bitunits, but signals can also be received in 4-bit or 2-bit units.

The answer modem confirms that the receive signal continues for 40 bitsor more (S12). If this is confirmed, the transmission of Flag signalswhich has been in progress so far is cut off, a turn-off sequence (all“1” signal is transmitted for 6 ms) is added and the transmissioncarrier is turned off (S12). (1) in FIG. 7 shows the period ofconfirming the reception of 40 bits or more and (2) shows the time atwhich the transmit signal carrier is turned off.

The answer modem then ignores the reception of all “1” signals from thecalling modem for a certain period of time (S14). This periodcorresponds to the delay on the line. Experimentally, 40 ms would be areasonable value.

By ignoring all “1” signals for a certain period of time in this way, itis possible to prevent a switchover to the primary channel when thereceive gain is drastically changed due to extinction of an echo whenthe transmission carrier is stopped, causing this change to be mistakenfor the stoppage of all “1” signals by the calling modem. In otherwords, this period is a standby period in which the AGC operation of thereceive circuit catches up with a drastic change of the amplitude of theincoming signal due to the stoppage of the transmission carrier andwaits until the all “1” signal received is confirmed to be stabilizedenough. This period corresponds to period (3) in FIG. 7.

When it confirms “off” of the transmission carrier from the answermodem, the calling modem restarts the transmission of all “1” signals.On the other hand, the answer modem restarts the reception of all “1”signals and if the receive signal is not an all “1” signal (S15), itswitches from the reception of the control channel to the reception ofthe primary channel (S16). If the receive signal is all “1”, the answermodem continues the reception in control channel mode until a “non-1”signal is received. If at least a single “non-1” bit signal is found, itonly needs to switch to the reception of the primary channel.

In FIG. 7, (4) indicates the next period of the reception of all “1”signals. (5) indicates the time at which “non-1” signals are confirmed,then a switchover is made from the control channel to the primarychannel. The calling modem transmits primary channel signals 65 to 75 msafter an all “1” signal is stopped and starts the transmission of imagesignals.

In the embodiment above, the answer modem operates the AGC during thereception of the control channel. In the present invention, it is alsopossible to fix the AGC immediately before the transmission carrier isturned off, that is, stop the AGC operation and continue to fix the AGCuntil the next stoppage of reception and until the confirmation of all“1” signals after restarting and receive the control channels under thatcondition. This prevents mistaking of bits due to variations of thereceive level, securing the switching timing.

As explained above, when the pre-processing of image signal transmissionis carried out by the full-duplex modem and then this is switched to thehalf-duplex modem to continue image signal transmission, the modemcontrol method of the present invention stops the transmission carrierafter a prescribed number of unit signals are received from the callingmodem and carries out a switchover after confirming that the callingmodem stops unit signals in response to this. In this method, the answermodem stops the reception for a certain period of time after thetransmission carrier is stopped during which signal disturbance mayoccur and restarts the reception when the signal level is stabilizedafter a prescribed time, detects the time at which unit signals cease tobe transmitted consecutively and switches to the half-duplex modem. Thisassures a switchover from the full-duplex modem to the half-duplexmodem.

Especially when the system is compliant with the T.30 ANNEX-F using aV.34 facsimile modem, the unit signal becomes “1”. On the other hand,until all “1” signals are stopped from immediately before thetransmission carrier is stopped or until it is confirmed that signalshave become “non-1” signals, the answer modem sets the AGC circuit ofthe receive circuit to fixed mode, making it possible to preventmistaking of bits reliably and switch to the primary channel correctly.

Industrial Applicability

The modem control methods in the present invention are suitable forsending image information in facsimile transmissions and especiallysuitable for facsimile transmissions when a considerable amount of delayis introduced into the line.

What is claimed is:
 1. A modem control method, comprising the steps of:confirming whether unit signals received from the calling modem continuefor a prescribed number of bits or not; turning off the transmissioncarrier if said unit signals continue for a prescribed number of bits;holding the processing in standby mode for a certain period of timeafter the transmission carrier has been turned off; restarting theprocess after a certain period of time, receiving unit signals, andswitching from the reception of the control channel to the reception ofthe primary channel when the reception of unit signals is completed. 2.The modem control method according to claim 1, wherein the AGC circuitof the receive circuit is set to fixed mode from immediately before thetransmission carrier is turned off until the system holds the processingin standby mode.
 3. The modem control method according to claim 1,wherein the AGC circuit of the receive circuit is set to fixed mode fromimmediately before the transmission carrier is turned off until the endof the reception of unit signals is confirmed after the processing isrestarted.
 4. The modem control method according to claim 1, whereinsaid certain period of time is a period until the received unit signalsare confirmed to be stabilized enough.
 5. The modem control methodaccording to claim 1, wherein said certain period of time is setaccording to a delay on the line.
 6. The modem control method accordingto claim 1, which uses an ITU Recommendation V.34 facsimile modem and iscompliant with the ITU Recommendation T.30 ANNEX-F.
 7. The modem controlmethod according to claim 6, wherein unit signals are “1”s.
 8. A modemcontrol method which carries out the pre-processing of image signaltransmission in facsimile transmissions using a full-duplex modem andthen performs image signal transmission by a half-duplex modem, whereinif, upon termination of the full-duplex modem, a request for terminationfrom the calling modem to transmit unit signals made up of prescribedbits continuously, a prescribed number of said unit signals are receivedfrom the calling modem first, the transmission to the calling modem isstopped, the reception of said unit signals from the calling modem isignored for a certain period of time, then the reception of said unitsignals is restarted and the process is switched over to the receptionby the half-duplex modem when the reception of these unit signals iscompleted.
 9. A modem control method which uses an ITU RecommendationV.34 facsimile modem, carries out the pre-processing of image signaltransmission using the control channel of a full-duplex modem incompliance with the ITU Recommendation T.30 ANNEX-F, and carries outimage signal transmission by switching to the primary channel of ahalf-duplex modem, wherein upon reception of a termination requestsignal of at least 40 all “1” bits indicating a request for controlchannel termination from the calling modem, after confirming thatsignals received from the calling modem are all “1” signals with 40 bitsor more, the transmission to the calling modem is stopped, the receptionof all “1” signals from the calling modem is ignored, then the receptionof all “1” signals is restarted, and the process is switched over to theprimary channel after confirming the end of reception of those all “1”signals.
 10. The modem control method according to claim 9, which setsthe operation of the AGC circuit which adjusts the gain of the receivesignal to fixed mode from immediately before the confirmation ofreception of all “1” signals from the calling modem is ignored for acertain period of time until a switchover to the reception of theprimary channel is completed.
 11. A modem control apparatus, comprising:signal confirmation means for confirming whether a signal from thecalling modem is a unit signal or not; carrier-off indication means forindicating transmission carrier off based on the output from said signalconfirmation means; processing standby/restart indication means forcontrolling indications of processing standby and processing restartingbased on the output from the carrier off indication means; switchingindication means for switching from the reception of the control channelto the reception of the primary channel based on the output from thesignal confirmation means after restarting the processing.
 12. The modemcontrol apparatus according to claim 11, wherein signal confirmationmeans receives unit signals from the calling modem and confirms whetherthe unit signals continue for a prescribed number of bits.
 13. The modemcontrol apparatus according to claim 11, wherein in the case that theunit signals continue for a prescribed number of bits, the carrier offindication means indicates to turn off the transmission carrier.
 14. Themodem control apparatus according to claim 11, wherein after turning offthe transmission carrier, the processing standby/restart indicationmeans holds the processing in standby mode for a certain period of timeand restarts the processing after the elapse of said period.
 15. Themodem control apparatus according to claim 11, wherein after restartingthe processing, the switching indication means receives unit signals andswitches from the reception of the control channel to the reception ofthe primary channel when the reception of unit signals is completed.